The present invention relates to a washing apparatus, to a purification apparatus, to a synthesis apparatus, to a method of purifying a wash material, and to the use of a washing apparatus or a purification apparatus for purification and, finally, to the use of the target product obtained by the purification.
Ever higher demands are being made of the purity of products produced in the chemical industry. This is true especially for so-called ‘fine chemicals’ or pharmaceutical products, which are produced in relatively small amounts. It has likewise been possible, relatively recently, for the trend towards ever higher purity requirements also to be seen in the case of chemicals that are produced in very large amounts. These substances are, for the most part, starting materials that are used in the further synthesis of mass-produced polymers. In this context there may be mentioned, for example, acrylic acid, methacrylic acid, styrene, acrylamide, caprolactam, naphthalene, or phenol. The high purity requirements are applicable especially when the products produced from those starting materials are used in the medical, food or hygiene sectors. By way of example of applications in the medical and hygiene sectors there may be mentioned absorbent polymers based substantially on partially neutralised cross-linked polyacrylic acids, which are used both in the medical and the hygiene sectors in the form of wound dressings or diapers, respectively, for absorbing aqueous body fluids. In the food sector, for example, flocculants based on acrylic acid or acrylamide or both are used in drinking-water treatment.
Another reason for the purity requirements applied to starting materials for polymerisation processes is that the polymerisation processes proceed in a substantially more controlled manner if the monomers used are present in a high degree of purity. It is possible, by that means, for the molecular weights and molecular weight distributions, which are crucial to the properties of the polymers synthesised from the monomers, to be better controlled.
A further area wherein high purity requirements are applied on an industrial scale is the field of waste-water treatment. The fact that organic solvents are increasingly being replaced by water or aqueous solvents in technical synthesis is resulting in ever greater amounts of waste water being produced from synthesis processes. However, the rules of environmental protection require that the products and by-products of the synthesis in question wherein water has been used as solvent be removed as completely as possible from the waste water.
In the food sector, the requirements applied to the purity of products intended for consumption are also ever increasing. This is true especially in the case of concentrate production. Also, for that purpose, there is a great need for economical concentration processes that are not detrimental to the food.
Likewise, because of the further technical development of heating systems, jet engines and internal combustion engines, the use of ever purer fuels is necessary for performance and exhaust gas optimisation.
However, when a chemical compound is synthesised or when a substance is obtained from natural sources, the desired substance is usually not in the form of a pure product. On the contrary, when carrying out synthesis or when obtaining a substance from natural sources, there is produced a mixture of compounds, of which part constitutes the desired substance, together with impurities such as solvents, starting materials and by-products or undesirable isomers. In order to separate off the desired substance from the impurities, distillative separation methods are frequently used on an industrial scale; however, these are associated with high energy use and, in the case of thermally sensitive and usually reactive end products, result in a lowering of the yield, because the desired products react further, owing to the relatively severe thermal conditions.
When the desired substance is a compound which can be crystallised and is present in a liquid mixture of compounds after the synthesis process, melt crystallisation is to be recommended as a very largely gentle method of purifying the desired substances, that is to say of separating out the substance from the liquid mixture of compounds, often referred to as the “feed”, in which further by-products are present in dissolved or liquid form. Thereby, the desired compound is crystallised out from the liquid that is referred to as the mother liquor, in the form of a solid, which is separated and re-melted. The melt is then taken off in the form of a pure product.
Customary methods of crystallisation known from the prior art are static and dynamic layer crystallisation, in which the compound to be isolated is precipitated on stationary cooled surfaces, or suspension crystallisation, which is based on the growth of crystals in a suspension. Suspension crystallisation has the advantage over layer crystallisation that it can be carried out in a continuous process. In addition, the purity of the crystals is generally very high because of their comparatively slow rate of growth. However, despite the slow rate of growth, a high product throughput rate can be achieved using suspension crystallisation because, for crystallisation in solution, a comparatively large area, namely the entire surface area of all the individual particles, is available for crystal growth. Suspension crystallisation consequently represents a very effective and economical method of achieving high purity levels in a target product.
Because of the relatively slow growth of the crystals, compared to layer crystallisation, the impurities present in the liquid are to a very large extent not incorporated into the crystal lattice and remain behind in the mother liquor. Even in a single-stage crystallisation process, high-purity crystals of the desired compound are generally obtained.
A further step that is important for the purity of the end product is the separation of the crystals present in the suspension from the other, liquid constituents of the suspension, which for the most part comprise impurities and the non-crystallised portions of the mixture to be purified. That separation is usually performed by means of a solid/liquid separation process. That separation may proceed in one or more stages, wherein at least in the final stage a so-called washing column is usually used as a washing apparatus. In such a washing apparatus, a suspension of crystals produced in a crystalliser is introduced and the suspension of crystals is compacted to form a crystal bed. A washing liquid, preferably the melt comprising the melted crystals themselves, is passed through that crystal bed in a contraflow direction.
Various methods are used for formation of the compact crystal bed. For example, U.S. Re. 24,038 discloses a gravimetrically operating washing column, wherein a crystal suspension is introduced in an upper region of the washing column and the crystal bed forms by virtue of a sedimentation process. In such columns there is, however, a risk that, in the course of the sedimentation process, vertical channels will form in which back-mixing of the mother liquor or crystal suspension with the washing liquid occurs.
As DE OS 1947251 discloses, gravimetrically operating washing columns have been equipped with a stirring mechanism, at least over part of their height, which prevents the formation of vertical liquid channels in the crystal bed. Such stirring mechanisms are, however, associated with the disadvantage that, because of the stirring movement, swirling-up of the crystal bed occurs, which has a disadvantageous effect on the separation performance of the washing apparatus.
Such stirring mechanisms are not required in the case of hydraulic or mechanical washing columns. For example, EP 0 920 894 A1 discloses a hydraulic washing column in which the suspension is conveyed under pressure into a pressure-tight housing and, as a result of the delivery pressure, forms a compacted crystal bed. The pressure is produced in EP 0 920 894 A1 by means of a semi-permeable piston, which is permeable to the liquid phase of the crystal suspension. In DE OS 28 00 540, a rotating conveying element, which is of helical construction, is used as a means of compacting the crystals in order to form the crystal bed.
In order to break-up again the crystals compacted in the crystal bed for supplying them to the region in which the crystals are melted, DE 100 39 025 A1 suggests a rotating removal tool which is arranged opposite the build-up face and which is generally in the form of a rotor blade or scraper.
It is common to the previously described movable means of conveying and compacting the crystals into a crystal bed and of separating off the crystals that they result in a non-homogeneous crystal bed. In addition to the significant costs incurred in incorporating such moving parts into the washing columns, a considerable outlay on maintenance is necessary. Maintenance of the moving parts in the washing column regularly results in operation of the washing column having to be suspended and the parts having to be dismantled, cleaned, repaired or replaced. A further disadvantage of moving parts in the washing column occurs especially during the purification of reactive substances. For example, when purifying acrylic acid, undesirable spontaneous polymerisation often occurs in the region of the seals of the shafts of the movable parts, resulting in operation of the washing column having to be suspended, in the polymers formed having to be removed and in the moving part having to be dismantled, repaired or replaced.
Concerning this problem a washing column is known from DE 37 86 471 T2, which doesn't have any movable parts in the region of the crystal bed. Furthermore, DE 37 86 471 T2 discloses different approaches to reduce or even to avoid plugging or blocking of the inlet opening. However, these methods do not take into account the complex fluidic conditions prevailing in the crystal bed and in the crystal suspension as well as in the region of transition between crystal suspension and crystal bed. These conditions are described in Effect of Compressibility on Performance of Hydraulic Wash Columns, L. van Oord-Knol, O. S. L. Bruinsma, P. J. Jansen, AIChE Journal, July 2002, Col. 48, No. 7, pages 1478 et seqq., and in A general Control Strategy For Hydraulic Packed Bed Wash Columns, P. J. Jansens, O. S. L. Bruinsma, G. M. van Rosmalen, R. de Goede, Trans IchemE, Vol. 72, Part A, September 1994, pages 695 et seqq., as being directed to the safe operation of washing columns.